The invention concerns a water heater with a gas inlet for a fuel gas/air mixture, an inlet for a fluid to be heated, at least two combustion stages with catalytic combustion chambers traversed by the fuel gas/air mixture and surrounded at least partly by at least one fluid chamber filled with the fluid, and with an exhaust gas heat exchanger traversed in various chambers by the exhaust gas escaping from the combustion chambers. The second combustion stage is fashioned as a monolithic burner.
Water heaters of that type are known in heating construction and, for instance, serve to heat water for an apartment heating system and to safeguard, as the case maybe, the hot-water supply of these apartments via a further water-water heat exchanger. Prior flame burners have the disadvantage of a high harmful NO.sub.x emission. Known from DE 33 32 572 A1 is a catalytic burner featuring a lower emission of noxious matter.
This device according to DE 33 32 572 A1 possesses two separate air supplies feeding primary and secondary air before the first combustion stage respectively between the first and second combustion stages. This separate air supply at a 60/40 percent ratio is to assure the heat release at 50 percent each in both stages.
This water heater consists in its catalytic combustion stages of two identical monolithic burners embedded each between two heat exchangers, with metal grids intended to prevent a flashback. Additionally, an uncoated ceramic element is arranged between said metal grid and the monolithic burner; it serves to prevent flashbacks and open combustion outside the combustion space proper.
This device has a number of disadvantages. For one, it requires an accurate air control for distribution of the supplied primary air and secondary air amount. Such a control with the additionally necessary piping complicates the structure of the water heater. While this design achieves gas compositions which avoid the generation of a critical temperature in the combustion chambers, the arrangement of the ceramic plate and metal grid does not prevent the flame operation between the two ceramic bodies with and without catalyst; on the contrary, even a heavy increase of the overall pressure drop of the burner occurs.
Due to the arrangement of the heat exchangers and to the quasi adiabatic operation, only the convective part is used in utilizing the heat energy generated. Due to the poor thermal conductance of the ceramic monoliths, which are exposed each to about 50 percent of the heat generated, so-called hot spots can occur in the monolithic burners, which may lead to premature aging of the catalysts.
Based on this prior art, the problem underlying the invention is to provide a water heater of the initially mentioned type which allows with a simple structure higher fuel utilization with lower emissions of noxious matter.
This problem is inventionally solved for a water heater according to the present invention in that the first combustion stage is fashioned as a catalytic gap burner, the combustion gap forming the combustion chamber of the first combustion stage and traversed by the gas mixture being bounded between a wall lined with a ceramic layer on the side facing the fluid chamber and a side coated with a catalyst layer, and in that the gap width is predetermined, in contingence on the flow velocity given by the gas throughput, in such a way that the flashback velocity is lower than the said flow velocity. By using a dual stage catalytic burner with stages configured for different heat capacities it is possible to convert the fuel gas without any residue, so that neither the fuel gas nor an NO.sub.x share can be detected in relevant magnitude in the exhaust gas leaving the water heater. Owing to the design of the combustion gap of the first stage between a wall lined with a ceramic layer on the side facing the fluid chamber and a side coated with the catalyst layer, it is possible to maintain the temperatures of 800.degree. Celsius that are necessary for a high conversion rate and to assure at the same time a swift heat transfer to the fluid to be heated.
Flashback is effectively prevented by predetermining the gap width in contingence of the traverse flow velocity given by the gas throughput, in such a way that the flashback velocity is lower than the said traverse flow velocity.
With the fuel gas mixture, prior to introduction in the catalytic combustion gap, able to be passed in counterflow along the backside of the wall lined with the catalytic layer, for preheating of the mixture, said fuel gas mixture can be preheated by the heat released during the reaction, so that nearly ideal conversion temperatures prevail throughout the combustion gap.